Metal joining methods play prominent roles in metallic component fabrication technology because virtually all such products are fabricated by joining procedures that require joint integrity that equals or surpasses component integrity. This field is extremely broad, involving many disciplines in science and engineering, a myriad of metals, alloys, composites, ceramics, and design criteria. The solid state joining technologies represent large categories of manufacturing processes including, but not limited to, bonding, brazing, and welding. Typically, welding can include arc welding, electron beam welding, diffusion bonding, chemical vapor deposition, adhesive bonding, fusion bonding, soldering, brazing, and friction welding. Originally developed on an empirical basis, these methods are accepted in today's manufacturing practices, though both products and processes often fail to meet the design criteria.
The design criteria require a process that is capable of manufacturing a joint to a level of integrity that meets or exceeds component structural integrity when introduced to field applications. For a successful joining process, surfaces to be joined must be prepared so that they are compatible to each other, both chemically and mechanically. Even when satisfactory joining processes are devised, subsequent performance is often limited due to structural alterations that occur during the joining processes and concomitant degradation of engineering properties thereafter. Typical joining problems are associated with surface preparation, pressure, temperature, and filler metals and often require some means for monitoring any significant degradation in the product integrity. Historically both destructive and non-destructive evaluation (NDE) methods have been used to study structural integral integrity under varying manufacturing variables and for different joining processes: 1) solid state joining, 2) liquid state joining, 3) fusion joining, and 4) atomic joining. However, use of destructive methods is applied to study product integrity is time-consuming and cost-prohibitive and provides planar instead of volumetric view. Similarly, NDE methods have been used to a large number of parts but with limited successes.
The selection of a NDE method for inspecting welded, brazed, and/or soldered structures or monitoring the nature of the metal joining practices depends on a number of variables. Some of these are: 1) nature and location of discontinuities, 2) accessibility and environment, 3) type of materials, 4) detection capabilities, 5) skills and training of operators, and 6) economic considerations. Several NDE methods, including radiography, eddy current, penetrant inspection, thermal, and ultrasonic have been used. Each method's success varies. For example, conventional radiography is not suitable for detecting extremely small defects. Penetrant has been found useful for cracks open to the surface and does not provide any data on volumetric inspection. Similarly, ultrasonic offers both subsurface and volumetric inspections but if suffers from poor sensitivity when detecting tightly held bonded structures (difference between intimate contact and diffusion bond). A reliable NDE method for inspecting and monitoring metal joint structures and processes is desirable.
A comparatively new emerging NDE technology is known as Acoustic Sensor Testing (AST) that is based on Non-Linear Acoustics (NA). Non-Linear Acoustics provides the most suited and powerful means for evaluating the mechanical integrity of a part as NA is directly related to its interactions with mechanical and material properties of the part. NA may use low frequency that possesses low attenuation and diffraction and provides sensitivity comparable to high frequency. In that in contrast to conventional ultrasonic, which is sensitive to the presence of gross defects or open cracks, NA is sensitive is sensitive in terms of wave-propagation characteristics to micro-crack, micro-porosity, and/or any mechanical degradation. Typically, when a sinusoidal wave (tone-burst and/or gated or any other forms) propagates through a joined or solid medium, it distorts and generates a higher harmonics of the fundamental waveform, and resonant frequency shift mode as a result of the non-linearity of the propagation medium. Waveform distortion does not depend on frequency; both low frequency and high frequency generates distortion. The generation of higher harmonics due to presence of non-linearity of the propagation medium is known as NA. NA has been used extensively in power, aerospace, and automotive industries. Depending on the accessibility, one can deploy different combinations, including but not limited to driving the part in pulse-echo, through transmission, and whole body vibration using either gated sine wave (example 5 MHz with 5 cycles) or sweeping with burst sine wave. In all these case, Non-linearity Acoustic will be used. Currently, there is no industry standard for NA but there are enough materials available to those having ordinary skill in the art. Besides, ASTM E234-10 may also serve some basis for this approach, though analytical tools proposed in it is widely differ from the one proposed in this invention.
In light of the above-noted deficiencies in the art, it would therefore be desirable to provide improved NDE methods. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.